Make build scripts Python3 compatible

- The Windows, UWP, Android (on Windows) and Linux builds are
  tested with Scons 3.0 alpha using Python 3.
- OSX and iOS should hopefully work but are not tested since
  I don't have a Mac.
- Builds using SCons 2.5 and Python 2 should not be impacted.

compat.py

@@ -0,0 +1,31 @@
+import sys
+
+if sys.version_info < (3,):
+    def isbasestring(s):
+        return isinstance(s, basestring)
+    def open_utf8(filename, mode):
+        return open(filename, mode)
+    def byte_to_str(x):
+        return str(ord(x))
+    import cStringIO
+    def StringIO():
+        return cStringIO.StringIO()
+    def encode_utf8(x):
+        return x
+    def iteritems(d):
+        return d.iteritems()
+else:
+    def isbasestring(s):
+        return isinstance(s, (str, bytes))
+    def open_utf8(filename, mode):
+        return open(filename, mode, encoding="utf-8")
+    def byte_to_str(x):
+        return str(x)
+    import io
+    def StringIO():
+        return io.StringIO()
+    import codecs
+    def encode_utf8(x):
+        return codecs.utf_8_encode(x)[0]
+    def iteritems(d):
+        return iter(d.items())